It has recently become possible to provide wired transmission of full-motion digitized video programming, rather than still frame or limited video conferencing, without the use of dedicated leased, wide bandwidth circuits and T1 lines. In particular, it has become possible to provide for the cost effective routing and distribution of a large amount of video or other data at high speed and in real time using the existing copper plant. For example, U.S Pat. Nos. 5,341,474 and 5,371,532 to Gelman et al discuss a "store-and-forward" architecture and method for distributing information to subscribers and U.S. Pat. No. 5,247,347, issued to L. Litteral et al on Sep. 21, 1993 and assigned to the assignee of the present invention, discusses a method of providing video-on-demand (VOD) services with VCR type functional control of the programming.
In the systems discussed in these three patents, a menu of programming information is accessible at the subscriber's premises. The subscriber may transmit order requests via the PSTN to the independent information providers. Information programs may be accessed and transmitted to the subscriber directly from an information provider or through a video buffer located at a central office (CO) serving the subscriber.
The information provider transmits coded digital data over wideband PSTN to a central office. The data may be buffered at the central office for transmission over a POTS (plain old telephone service) line to the subscriber using a video gateway. An access concentrator/packet switch at the central office receives the order request from the subscriber loop. A video gateway in turn is responsive to the order request for producing routing data. A file server, responsive to requests received from the video gateway, obtains selected information from the digital information storage medium. A circuit or ATM cross-connect switch in turn is responsive to the routing data for transferring the information from the file server to the subscriber loop transmission means.
A subscriber in such a system may use either a standard telephone instrument over the PSTN or a dedicated control device over an X.25/ISDN packet network to order the information program. The dedicated control device is either a settop box which is located at a television set of the subscriber and which permits a display of the program menu on the television screen or a computer with appropriate control boards.
Connectivity between the central office and the subscriber for transmission of data is provided by one of a number of different interface units, such as an asymmetrical digital subscriber line (ADSL) interface or a high bit rate digital subscriber line (HDSL) interface. ADSL systems are generally preferable to bi-directional HDSL systems for services such as video-on-demand in which the amount of information transmitted in one direction is much greater than the amount of information transmitted in the opposite direction. ADSL systems provide a cost effective and bandwidth efficient platform for receiving or transmitting a large volume of information at high speed in a single direction using the existing copper plant. An asymmetrical communication transport provides high bandwidth for fast transfer of bulk data in one direction, while a slow transmission rate is used for bi-directional communication. Typically, a request for data requires very little data transfer in the upstream direction and the data delivery involves high data traffic in the downstream direction. In addition, ADSL systems are generally capable of transmitting over longer distances than HDSL, do not interfere with POTS or ISDN communications and therefore require only a single copper pair.
T1 communication enables simultaneous high speed digital transmission of bulk data in both directions, but requires special wiring and circuitry and exceeds requirements for data network applications in which high bulk data transfer occurs only in one direction at a time. Existing video conferencing transceivers and HDSL systems permit bi-directional transmission at high bit data rates.
The data rates of the channels in conventional ADSL systems are typically predetermined according to the structure of the ADSL interface units. Therefore, although previous systems may be well suited for some services such as video-on-demand, they are not well suited for other services in which the nature and amount of data and control signal transfer is substantially different or changes frequently. Specifically, the two-way control channel may be unacceptably slow for services such as interactive multi-media, distance learning, or accessing a server in a remote local area network (LAN) over a POTS line using a single copper pair. One or more of these services may require a bi-directional control channel of up to, for example, 384 kbps in order to allow substantially real-time communications so that a subscriber is not waiting for information to be transmitted.
Furthermore, systems which provide services such as video-on-demand and home shopping typically only provide for the transfer of video programming or other data in a single direction from a VIP or other information provider to a subscriber. They may not provide significant bi-directional capability, full connectivity amongst all of the subscribers on the network, or multi-cast transmission capability which can be point-to-point or point-to-multipoint for distance learning, video phone etc.